Re: Destroyed table with `tabs -4`

[Thomas Dickey]

On Wed, Feb 13, 2019 at 09:54:54PM +0100, Frank Brütting wrote:
> Hi,
> 
> I have `tabs -4` in my `.bashrc`, because that’s what is used everywhere.
> Sadly however, this destroys the output of all applications which use
> `ncurses`.

But it's not a bug in ncurses.  Very briefly:

The tabs command can set hardware tab-stops (those need not be the same
size).  ncurses can be configured to use hardware tab-stops (and by the
way, that's not a default setting).

These items in terminfo(5) are relevant:

       init_tabs                     it         it        tabs initially every
                                                          # spaces
       clear_all_tabs                tbc        ct        clear all tab stops
                                                          (P)
       set_tab                       hts        st        set a tab in every
                                                          row, current columns
       tab                           ht         ta        tab to next 8-space
                                                          hardware tab stop

While the information needed to _set_ tab-stops is in the terminal
database, that tells curses applications only what the _initial_
tab-size is.  There's no completely portable way for curses to determine
what the tabs program did.

When you used

        tabs -4

that made the terminal behave differently (from curses standpoint) than the

        it#8

value in the terminal description.  curses (SVr4 curses, ncurses, NetBSD)
all provide a library variable TABSIZE which an application could set to
reflect a tab-size which was set externally.  Otherwise, they all use the
initial-tabs value from the terminal database.  There's no environment
variable for this in SVr4 curses or ncurses.  With a little imagination,
you can see how an environment variable for it would be more trouble than
it's worth.

That's been documented for quite a while (July 2000 in curs_addch.3x,
September 2010 in curs_variables.3x).  I see some places to improve
the documentation, but no code or database change is needed.

You could use a custom terminal description for this, with

        it#4

For anything more involving varying tab-sizes
(the tabs manual page shows several),
there's no curses (or similar) library that does this.
 
> See my more detailed bug report here, for example:
> https://github.com/gnunn1/tilix/issues/1616#issuecomment-453881648

I see (nothing useful there).

-- 
Thomas E. Dickey <address@hidden>
https://invisible-island.net
ftp://ftp.invisible-island.net

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